1. Field of the Invention
The present invention relates to a display device, particularly, to an OLED display device using a thin film transistor formed on the transparent substrate such as glass or plastic. Further, it relates to an electronic device using the display device.
2. Description of the Related Art
In resent years, a cellular phone has been becoming popular by developing communication technology. In future, electrical transmission of moving pictures and transmission of a large quantity of information will be expected. With being lightened, a mobile personal compute is into production. An information device called a personal digital assistant (PDA) developed from electrical books is produced and becoming popular. With developing a display device and the like, most of such portable information devices are equipped with flat displays.
The latest technology aims at using an active matrix display device as a display device used in the portable information device.
In the active matrix display device, TFTs (thin film transistors) are provided in correspondence with respective pixels to control pictures. The active matrix display device has an advantages that the high definition of images is possible, the improvement of image quality is possible, the correspondence to moving image is possible, and the like, compared to a passive matrix display device. Therefore, the display device of the portable information device will be changed from a passive matrix type to an active matrix type.
Above all, a display device using low-temperature polysilicon has been production in recent years. In the low-temperature polysilicon technology, the driver circuit using TFTs can be formed simultaneously in the periphery of a pixel portion in addition to a pixel TFT that constitutes a pixel. Thereby, the low-temperature polysilicon technology can contribute to miniaturization of devices and low power consumption. Accordingly, the low-temperature polysilicon device become indispensable to the display device of the mobile device which has been widely applied to various fields in recent years.
In recent years, the development of a display device using an organic electro luminescence elements (OLED elements has been becoming more and more active. Hereinafter, the OLED element includes both the OLED element using luminescence from singlet exciton (fluorescence) and the OLED element using luminescence from triplet exciton (phosphorescence) here. In this specification, the OLED element is described as an example of a light emitting element, however, another light emitting elements can be used.
The OLED element has a structure in which an OLED layer is interposed between a pair of electrodes (an anode and a cathode), and usually has a laminated structure. Representatively, there is a laminated structure which is called “hole transporting layer/light emitting layer/electron transporting layer”, proposed by Tang et al. of Kodak Eastman Company.
Other structures may also be adopted, such as a structure in which “a hole injecting layer, a hole transporting layer, a light emitting layer and an electron transporting layer” are stacked on an anode in order, or a structure in which “a hole injecting layer, a hole transporting layer, a light emitting layer, an electron transporting layer and an electron injecting layer” are laminated on an anode in order. The light emitting layer may also be doped with a fluorescent pigment or the like.
In this specification, all layers provided between a cathode and an anode are herein generically called “OLED layer”. Accordingly, all the aforementioned hole injecting layer, hole transporting layer, light emitting layer, electron transporting layer and electron injecting layer are encompassed in the OLED layer. A light emitting element constituted of an anode, an OLED layer, and a cathode is called “OLED element”.
FIG. 5 shows an example of the construction of a pixel portion of an active matrix type OLED display device. A gate signal line (G1 to Gy) to which a selection signal is to be inputted from a gate signal line driver circuit is connected to a gate electrode of a switching TFT 301 which is provided in each pixel of the pixel portion. Either one of source and drain regions of the switching TFT 301 provided in each pixel is connected to a source signal line (S1 to Sx) to which a signal is to be inputted from a source signal line driver circuit, while the other is connected to a gate electrode of an OLED driving TFT 302 and to either one of electrodes of a capacitor 303 which is provided in each pixel. The other electrode of the capacitor 303 is connected to a power supply line (V1 to Vx). Either one of source and drain regions of the OLED driving TFT 302 provided in each pixel is connected to the power supply line (V1 to Vx), while the other is connected to one of electrodes of the OLED element 304 provided in each pixel.
The OLED element 304 has an anode, a cathode and an OLED layer provided between the anode and the cathode. If the anode of the OLED element 304 is connected to the source region or the drain region of the OLED driving TFT 302, the anode and the cathode of the OLED element 304 become a pixel electrode and a counter electrode, respectively. Contrarily, if the cathode of the OLED element 304 is connected to the source region or the drain region of the OLED driving TFT 302, the cathode and the anode of the OLED element 304 become a pixel electrode and a counter electrode, respectively.
Incidentally, the potential of the counter electrode is herein called “counter potential”, and a power source for applying the counter potential to the counter electrode is herein called “counter power source”. The difference between the potential of the pixel electrode and the potential of the counter electrode is an OLED driving voltage, and the OLED driving voltage is applied to the OLED layer.
As a gray scale display method for the above-described EL display device, there are an analog gray scale method and a time gray scale method.
First, the analog gray scale method for the OLED display device will be described below. FIG. 6 is a timing chart showing the case driving the display device shown in FIG. 5 by the analog gray scale method. The period that starts when one gate signal is selected and finishes when the next gate signal line is selected is herein called “one line period (L)”. The period that starts when one image is selected and finishes when the next image is selected corresponds to one frame period. In the case of the OLED display device shown in FIG. 5, the number of gate signal lines is “y”, and y-number of line periods (L1 to Ly) are provided in one frame period.
As resolution of the OLED display device becomes higher, the number of line periods for one frame period becomes larger, and the driver circuit of the OLED display device must be driven at a higher frequency.
The power source lines (V1 to Vx) are kept at a constant voltage (power source potential). In addition, the counter potential is kept constant. The counter potential has a potential difference from the power source potential so that the OLED elements emit light.
In the first line period (L1), a selection signal from the gate signal line driver circuit is inputted to the gate signal line G1. Then, analog video signals are inputted to the source signal lines (S1 to Sx) in order.
Since all the switching TFTs 301 connected to the gate signal line G1 are turned on, the analog video signals inputted to the source signal lines (S1 to Sx) are respectively inputted to the gate electrodes of the OLED driving TFTs 302 via the switching TFTs 301.
According to the potential of the analog video signal inputted into the pixel when the switching TFT 301 is turned on, the gate voltage of the OLED driving TFT 302 varies. At this time, the drain current of the OLED driving TFT 302 to the gate voltage is determined at a 1-to-1 ratio in accordance with the Id-Vg characteristic of the OLED driving TFT 302. Specifically, according to the potential of the analog video signal inputted to the gate electrode of the OLED driving TFT 302, the potential of the drain region of the OLED driving TFT 302 (an OLED driving voltage which is corresponding to the on state) is determined, a predetermined drain current flows into the OLED element 304, and the OLED element 304 emits light at the amount of emission which is corresponding to the amount of the drain current.
When the above-described operation is repeated until the termination of inputting the analog video signals to the respective source signal lines (S1 to Sx), the first line period (L1) terminates. Incidentally, one line period may also be defined as the sum of the period required until the termination of inputting the analog video signals to the respective source signal lines (S1 to Sx) and a horizontal retrace period. Then, the second line period (L2) starts, and a selection signal is inputted to the gate signal line G2. Similarly to the first line period (L1), analog video signals are inputted to the source signal lines (S1 to Sx) in order.
When selection signals are inputted to all the gate signal lines (G1 to Gy), all the line periods (L1 to Ly) terminate. When all the line periods (L1 to Ly) terminate, one frame period terminates. During one frame period, all the pixels perform displaying and one image is formed. Incidentally, one frame period may also be defined as the sum of all the line periods (L1 to Ly) and a vertical retrace period.
As described above, the amount of emission of the OLED element is controlled by the analog video signal, and gray scale display is provided by controlling the amount of emission. In the analog gray scale method, gray scale display is carried out by the variation in the potentials of the respective analog video signals inputted to the source signal lines.
The time gray scale method will be described below.
In the time gray scale method, digital signals are inputted to pixels to select a emitting state or a non-emitting state of the respective OLED elements, whereby gray scales are represented by accumulating periods per frame period during which each of the OLED elements emits.
In the following description, 2n gray scales (n is a natural number) are represented. FIG. 7 is a timing chart showing the case of driving the display device shown in FIG. 5 by the time gray scale method. One frame period is divided into n-number of sub-frame periods (SF1 to SFn). Incidentally, the period for which all the pixels in the pixel portion display one image is called “one frame period (F)”. Plural periods into which one frame period is divided are called “sub-frame periods”, respectively. As the number of gray scales increases, the number into which one frame period is divided also increases, and the driver circuit of the OLED display device must be driven at a higher frequency.
One sub-frame period is divided into a write period (Ta) and a display period (Ts). The write period is a period for which digital signals are inputted to all the pixels during one sub-frame period, and the display period (also called “lighting period”) is a period for which the respective OLED display devices are in an emitting state or a non-emitting state in accordance with the input digital signals, thereby perform displaying.
The OLED driving voltage shown in FIG. 7 represents the OLED driving voltage of an OLED element for which the emitting state is selected. Specifically, the OLED driving voltage of the OLED element for which the emitting state is selected is 0 V during the write period, and has a magnitude which enables the OLED element to emit light, during the display period.
The counter potential is controlled by an external switch (not shown) so that the counter potential is kept at approximately the same level as the power source potential during the write period, and has, during the display period, a potential difference from the power source potential to so that the OLED element emits light.
The write period and the display period of each sub-frame period will first be described in detail with reference to FIGS. 5 and 7, and subsequently, the time gray scale method will be described.
First, a gate signal is inputted to the gate signal line G1, and all the switching TFTs 301 connected to the gate signal line G1 are turned on. Then, digital signals are inputted to the source signal lines (S1 to Sx) in order. The counter potential is kept at the same level as the potential of the power supply lines (V1 to Vx) (power source potential). Each of the digital signals has information of “0” or “1”, that is, each of the digital signals of “0” or “1” has a voltage of high level or low level.
Then, the digital signals inputted to the source signal lines (S1 to Sx) are respectively inputted to the gate electrodes of the OLED driving TFTs 302 via the switching TFTs 301 which are in the on state. The respective digital signals are also inputted to the capacitors 303 and stored.
Then, the above-described operation is repeated by inputting gate signals to the respective gate signal lines (G2 to Gy) in order, whereby digital signals are inputted to all the pixels and the input digital signal is held in each of the pixels. The period required until the digital signals are inputted to all the pixels is called “write period”.
When the digital signals are inputted to all the pixels, all the switching TFTs 301 are turned off Thus, the external switch (not shown) connected to the counter electrode causes the counter potential to vary so that a potential difference that enables the OLED element 304 to emit light is produced between the counter potential and the power source potential.
In the case where the digital signals have information of “0”, the OLED driving TFTs 302 are turned off and the OLED elements 304 do not emit light. Contrarily, in the case where the digital signals have information of “1”, the OLED driving TFTs 302 are turned on. Consequently, the pixel electrodes of the respective OLED elements 304 are kept at approximately the same potential as the power source potential, and the OLED elements 304 emit light. In this manner, the emitting state or the non-emitting state of the OLED elements 304 is selected in accordance with the information of the digital signals, and all the pixels perform displaying at the same time. When all the pixels perform display, an image is formed. The period for which the pixels perform displaying is called “display period”.
The lengths of the write periods (Ta1 to Tan) of all the n-number of sub-frame periods (SF1 to SFn) are the same. The display periods (Ts) of the respective sub-frame periods (SF1 to SFn) are denoted by Ts1 to Tsn.
The lengths of the respective display periods are set to become Ts1:Ts2:Ts3: . . . :Ts(n-1):Tsn=20:2−1:22: . . . :2−(n-2):2−(n-1), respectively. By combining desired ones of these display periods, it is possible to provide a desired gray scale of 2n gray scales.
The display period is any one of Ts1 to Tsn. Here, it is assumed that predetermined pixels are turned on for Ts1.
Then, when the next write period starts and data signals are inputted to all the pixels, the next display period starts. At this time, the display period is any one of Ts2 to Tsn. Here, it is assumed that predetermined pixels are turned on for Ts2.
The same operation is repeated as to the remaining (n-2)-number of sub-frames, whereby the display periods are set as Ts3, Ts4, . . . , Tsn in order and predetermined pixels are turned on during each of the sub-frames.
When the n-number of sub-frame periods appear, one frame period terminates. At this time, the gray scale of a pixel is determined by cumulatively calculating the length of the display periods for which the pixel is turned on. For example, assuming that n=8 and the obtainable luminance in the case where the pixel emits light for all the display periods is 100%, a luminance of 75% can be represented if the pixel emits light during Ts1 and Ts2 and a luminance of 16% can be realized if Ts3, Ts5 and Ts8 are selected.
Incidentally, in the driving method of the time gray scale method which represents gray scales by inputting n-bit digital signals, the number of plural sub-frame periods into which one frame period is divided, and the lengths of the respective sub-frame periods and the like are not limited to the above-described examples.
The conventional OLED display device as described above has the following problems.
A voltage is supplied to a pixel from a source signal line, and the voltage is converted into a current by an OLED driving TFT. Thus, even if the same voltage is input, different currents flow to OLED elements due to dispersion of performance between the OLED driving TFTs, and the fact that luminance of the pixel differs in a different location causes unevenness of display. For example, when the thickness of a gate insulating film differs within a substrate, an on current of the OLED driving TFT differs in a different location. As a result, the emission luminance of the OLED element differs to cause unevenness of display. This defect is a more serious problem as a panel is increased in size.